Thermal Imaging as a Method to Indirectly Assess Peripheral Vascular Integrity and Tissue Viability in Veterinary Medicine: Animal Models and Clinical Applications

Abstract

Infrared thermography (IRT) is a technique that indirectly assesses peripheral blood circulation and its resulting amount of radiated heat. Due to these properties, thermal imaging is currently applied in human medicine to noninvasively evaluate peripheral vascular disorders such as thrombosis, thromboembolisms, and other ischemic processes. Moreover, tissular damage (e.g., burn injuries) also causes microvasculature compromise. Therefore, thermography can be applied to determine the degree of damage according to the viability of tissues and blood vessels, and it can also be used as a technique to monitor skin transplant procedures such as grafting and free flaps. The present review aims to summarize and analyze the application of IRT in veterinary medicine as a method to indirectly assess peripheral vascular integrity and its relation to the amount of radiated heat and as a diagnostic technique for tissue viability, degree of damage, and wound care.

Keywords: burn wounds; free flaps; ischemia; skin grafting; thrombosis; wound healing.

Figure 1

The surface temperature of a dog undergoing an infectious disease. (A) Thermal response in a healthy 3-year-old mixed female dog. The maximum (red triangle) surface temperature of the periocular region (El1) shows a value of 37 °C, while the minimum (blue triangle) and average temperatures were 36 °C and 35 °C, respectively. (B) Thermal response in a 7-year-old female English Setter dog diagnosed with pyometra. It is observed that the surface temperature of the periocular region (El1) is 1 °C higher than the maximum (red triangle) and average temperature of a healthy dog. Regarding the minimum temperature (blue triangle), a difference of +1.1 °C was found when compared to a healthy animal. The presence of cytokines, such as interleukin (IL)-1, IL-6, and prostaglandin F2 alpha, induces fever. The increase in body core temperature is reflected in the vasodilation of superficial blood vessels to increase heat dissipation. Therefore, IRT can help recognize febrile states in animals. Radiometric images were obtained using a T1020 FLIR thermal camera. Image resolution 1024 × 768; up to 3.1 MP with UltraMax. FLIR Systems, Inc. Wilsonville, OR, USA.

Figure 2

Thermal response associated with an inflammatory process in a dog with a knee fracture. (A) Thermal response in a 6-year-old female Shiba Inu dog with inflammation. Temperature differences can be observed in the right pelvic limb with a fracture in the femoral–tibial–patellar joint. The maximum surface temperature (red triangle) of the region (El1) was 35.2 °C, with an average temperature of 34.1 °C and a minimum of 33 °C (blue triangle). (B) Thermal response in a healthy knee. In the same animal, the thermal response of the femoral–tibial–patellar joint (El1) of the left hindlimb had a difference of up to 3.3 °C when comparing the average surface temperature of both knees. The release of pro-inflammatory mediators (e.g., IL-1, IL-6, IL-10, prostaglandins, serotonin, and histamine) produces vasodilation of capillaries in the dermal tissue, increasing heat dissipation and radiation.

Figure 3

Peripheral vascular changes due to ischemic and inflammatory processes. Whether a decreased or increased blood flow is present, changes in the local temperature arise due to alterations in the normal blood flow. For example, during thromboembolisms, initially, the thrombus decreases blood flow by obstructing part of the blood vessel. However, the vessel can be completely blocked when an embolus travels through the bloodstream. Locally, this will stop or significantly reduce blood flow, decreasing the local surface temperature. On the contrary, during an inflammatory process such as osteoarthritis, vasodilation of venules and arterioles increases blood flow to promote neutrophil emigration. The release of inflammatory mediators also causes vasodilation, increasing local heat radiation.

Figure 4

Difference in the thermal response of patients diagnosed with peripheral vascular alterations. (A) Persian male cat diagnosed with aortic thromboembolism after evaluating clinical signs such as pain on palpation in the right pelvic limb, absence of pulse, cold limb, and mobility difficulty. (B) The phalangeal region of the affected right hindlimb (El1) showed lower temperatures of up to 3.3 °C when compared to the healthy left hindlimb (El2). (C) A four-year-old male Dachshund dog diagnosed with thrombosis due to a secondary liver infection was presented with a reduced perfusion in the left forelimb. Necrosis can be observed. (D) With thermal imaging, it can be observed that the average surface temperature of the phalangeal region of the right forelimb (El2) is 5.2 °C higher than the same region in the affected limb (El1). The explanation for this thermal response is that the presence of a thrombus obstructs blood flow due to occlusion at the arterial level. Thus, the decrease in blood flow has an impact on local heat response. Maximal temperature is indicated with a red triangle and the minimal with a blue triangle. Radiometric images were obtained using a T1020 FLIR thermal camera. Image resolution 1024 × 768; up to 3.1 MP with UltraMax. FLIR Systems, Inc. Wilsonville, OR, USA.